ΔE 9.4, close: similar, with a visible difference side by side. IJA Grey Green is a little lighter, more muted, cooler than Thar Brown.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). IJA Grey Green is a little lighter, more muted, cooler than Thar Brown.
The same difference applies the other way round: Thar Brown is a little darker, more saturated, warmer.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Thar Brown on the colour wheel See IJA Grey Green on the colour wheel
All matches: Thar Brown equivalents · IJA Grey Green equivalents